In a conventional method for recognizing patterns, the object patterns are compared and collated with template patterns previously registered, and a decision is made that the template pattern having the least degree of mismatching (or most degree of matching) is identified as the object pattern. In semiconductor wafer processing, pattern recognition is used, e.g., for wafer alignment. A pattern recognition method typically involves memorizing the pattern of a specified characteristic area, i.e. the key pattern, on the surface of a semiconductor wafer located at a predetermined position, and the position of the key pattern, and detecting the same pattern as the key pattern at other locations on the surface of the semiconductor wafer.
By way of example, U.S. Pat. No. 4,757,550 describes an automatic alignment system that uses pattern recognition. This system performs primary positioning of an object on the basis of low magnification pattern matching of a part of the image. The system subsequently performs secondary positioning of the object on the basis of high magnification pattern matching of a part of the image.
U.S. Pat. No. 4,672,678 describes a pattern recognition apparatus. The apparatus includes an image input device, an analog-to-digital converter that converts an analog image signal from the input device to a binary signal. A frame memory stores the binary signal. A pattern recognition circuit compares the binary signal stored in frame memory with a template memory. A template matching circuit compares character and image patterns in the template memory. A featured template forming circuit automatically forms mismatched portions of the character and image patterns as a featured template and registers them in the featured template.
U.S. Pat. No. 4,115,762 describes an alignment pattern detecting apparatus having an image pick-up device that scans and picks up an optical image of alignment pattern formed on a wafer to produce a time-base video signal. The video signal is sampled at predetermined intervals and converting from analog to digital form so that the video signal is returned symmetrically at a predetermined point to determine the degree of matching between the two signals to obtain a point where the degree of matching is the best. This best matching degree point is detected as the center position of the alignment pattern.
The above prior art techniques were capable of performing pattern recognition on high contrast wafers using bright field systems. Unfortunately, when a wafer has a thicker and more opaque film, the pattern recognition images generally show a low pattern contrast. Such, low contrast wafers present a challenge for prior art pattern recognition techniques. Bright field images from low contrast wafers provide very few characters (or features) in binary signals. These image features can appear differently when imaged with different systems. The variations between images taken with two systems are largely a function of system alignment. Prior art methods of pattern recognition are not robust enough to handle trivial deviation of low contrast images between systems. These trivial variations can lead to failure of pattern recognition matching.
Thus, there is a need in the art, for a pattern recognition method and system that overcomes the above disadvantages.